The present invention relates to organic electroluminescent (EL) devices, also known as organic light-emitting diodes (OLED) that emit color light.
In color or full-color organic electroluminescent (EL) displays (also known as organic light-emitting diode devices, or OLED devices) an array of colored pixels is provided. These pixels can include red, green, and blue color pixels (commonly referred to as RGB pixels). These pixels are precision patterned. The basic OLED device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer or light-emitting layer of the organic EL medium. Other organic layers present in the organic EL medium can provide charge transport functions primarily and are referred to as either the hole-transporting layer (for hole transport) or electron-transporting layer (for electron transport). In forming the RGB pixels in a full-color OLED display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material is deposited on a substrate in a selected pattern via the use of a donor coating on a support and an aperture mask.
The EL material transfer is preferably done under conditions of reduced oxygen and/or water, using a chamber such as Tang describes in the aforementioned patent. The use of vacuum or reduced pressure can facilitate the transfer of the EL material from the source to the substrate. The use of such conditions during transfer is also advantageous in that some EL materials are sensitive to oxygen and/or moisture. For example, tris(8-quinolinolato)-aluminum(III) (Alq), which is used in OLED devices, is known to react with water [F. Papadimitrakopoulos et al, Chem. Mater. 8, 1363 (1996)]. In addition, many electrode materials used on both small molecule and polymer EL devices are extremely unstable in air. The use of a vacuum or low oxygen and/or water conditions during the transfer step can help reduce the failure rate of OLED devices. However, inadvertent contamination of the device by oxygen, moisture, and/or other components is possible during or between deposition steps or anytime there is an apparatus transfer or delay between steps. This can lead to reduced stability of OLED displays. In addition, radiation transfer of organic light-emitting materials can be a high energy process that can reduce the stability of OLED displays made by this method.
It is therefore an object of the present invention to improve the stability of an OLED device.
This object is achieved by a method for forming an organic light-emitting device with improved performance comprising the steps of:
a) forming an anode over a substrate;
b) providing a donor element including light-emitting materials and positioning such donor element in a material-transferring relationship with the substrate;
c) illuminating the donor element with radiation to cause the transfer of light-emitting material to deposit the light-emitting material and form a light-emitting layer over the anode;
d) forming a performance-enhancing layer over the light-emitting layer including one or more chemical reducing materials selected to improve the performance of the organic light-emitting device;
e) forming an electron-transporting layer over the performance-enhancing layer; and
f) forming a cathode over the electron-transporting layer.
It is an advantage of this invention that an OLED device manufactured by radiation transfer of a light-emitting material has improved operational stability.